Page 249 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART III Basic Virology
238
VIR AL MESSENGER RNA
There are four interesting aspects of viral mRNA and its
expression in eukaryotic cells. (1) Viral mRNAs have three
transcribed, and therefore, only certain early proteins are
made. One of the early proteins is a repressor of the late
attributes in common with cellular mRNAs: on the 5´ end
there is a methylated GTP “cap,” which is linked by an
“inverted” (3´-to-5´) bond instead of the usual 5´-to-3´
(4) Three different processes are used to generate the mono-
cistronic mRNAs that will code for a single protein from the
bond; on the 3´ end there is a tail of 100–200 adenosine resi- genes; this prevents transcription until the appropriate time.
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polycistronic viral genome:
dues [poly(A)]; and the mRNA is generated by splicing from
(1) Individual mRNAs are transcribed by starting at many
a larger transcript of the genome. In fact, these three modifi-
cations were first observed in studies on viral mRNAs and
then extended to cellular mRNAs. (2) Some viruses use their
same mechanism used by eukaryotic cells and by herpesvi-
genetic material to the fullest extent by making more than
ruses, adenoviruses, and the DNA and RNA tumor viruses.
one type of mRNA from the same piece of DNA by “shifting
(2) In the reoviruses and influenza viruses, the genome is
segmented into multiple pieces, each of which codes for a
the reading frame.” This is done by starting transcription one
single mRNA.
or two bases downstream from the original initiation site.
(3) In polioviruses, the entire RNA genome is translated
(3) With some DNA viruses, there is temporal control over
the region of the genome that is transcribed into mRNA.
proteins by a protease.
During the beginning stages of the growth cycle, before DNA into one long polypeptide, which is then cleaved into specific
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A. Poliovirus
Viral genome is the mRNA
Translation
Full-length precursor
polypeptide (polyprotein)
Cleavage by virus-encoded protease

P
P
P
3
2
1
Further cleavage
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Capsid proteins and enzymes
B. Retroviruses
Integrated DNA copy
of viral RNA genome
Transcription

mRNA for Gag-Pol polyprotein
Translation

Gag-Pol polyprotein

Cleavage by virus-encoded protease
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Capsid proteins and enzymes
FIGURE 29–4
Synthesis of viral precursor polypeptides. A: Poliovirus mRNA is translated into a full-length precursor polypeptide, which is
cleaved by the virus-encoded protease into the functional viral proteins. B: Retroviral mRNAs are translated into precursor polypeptides, which
are then cleaved by the virus-encoded protease into the functional viral proteins. The cleavage of the Gag-Pol precursor polyprotein by the virion
protease occurs in the immature virion after it has budded out from the cell membrane. The cleavage produces the capsid protein (p24), the
matrix protein (p17), and enzymes such as the reverse transcriptase and the integrase. The cleavage of the Env polyprotein is carried out by a cel-
lular protease, not by the virion protease. Inhibitors of the virion protease are effective drugs against human immunodeficiency virus.

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